The present invention relates to a photomask used for forming a fine pattern in fabrication of semiconductor integrated circuit devices and a pattern formation method using the photomask, and further relates to a design method for a mask pattern.
Recently, there are increasing demands for refinement of circuit patterns in order to increase the degree of integration of a large scale integrated circuit (hereinafter referred to as the LSI) realized by using semiconductors. As a result, thinning of an interconnect pattern used for forming a circuit has become very significant.
Now, conventional thinning of an interconnect pattern using an optical exposure system will be described by exemplifying use of positive resist process. Herein, a line pattern means a portion of a resist film not exposed to exposing light, that is, a resist portion remaining after development (namely, a resist pattern). Also, a space pattern is a portion of the resist film exposed to the exposing light, that is, an opening where the resist film is removed through the development (namely, a resist removal pattern). In the case where negative resist process is employed instead of the positive resist process, the above-described definitions of a line pattern and a space pattern are replaced with each other.
Conventionally, in the case where pattern formation is carried out by using an optical exposure system, a photomask in which a completely shielding pattern of Cr or the like is drawn correspondingly to a desired pattern on a transparent substrate of quartz or the like is used. In such a photomask, an area where the Cr pattern is present works as a shielding portion that does not transmit exposing light of a given wavelength at all (namely, that has transmittance of substantially 0%), and an area where the Cr pattern is not present (an opening) works as a transparent portion that has transmittance equivalent to that of the transparent substrate against the exposing light (namely, that has transmittance of substantially 100%). When this photomask is used for the exposure, the shielding portion corresponds to an unexposed portion of a resist and the opening (transparent portion) corresponds to an exposed portion of the resist. Accordingly, such a photomask, namely, a photomask consisting of a shielding portion and a transparent portion against exposing light of a given wavelength, is designated as a binary mask.
In an optical exposure system, an image formed through exposure using the above-described binary mask (namely, an energy intensity distribution caused on a target material through the exposure) has contrast in inverse proportion to λ/NA, wherein λ indicates the wavelength of the exposing light emitted from a light source and NA indicates numerical aperture of a reducing projection optical system (specifically, a projection lens) of an aligner. Therefore, a dimension that can be formed as a resist pattern is in proportion to λ/NA. Accordingly, in order to realize refinement of a pattern, it is effective to reduce the wavelength λ of the exposing light and increase the numerical aperture NA.
On the other hand, for example, since steps are caused in formation of devices included in an LSI or the surface of a substrate is not flat, an image formed by using the conventional optical exposure system may be shifted from an ideal focal point. Therefore, the dimension of a pattern formed in such a defocused state should be kept in a predetermined range. The limit of a defocus value at which the dimension of a pattern falls within the predetermined range, namely, the limit of a defocus value for securing the dimensional accuracy of a pattern, is designated as a depth of focus (DOF). Specifically, in order to refine a pattern, it is necessary not only to emphasize the contrast of an image but also to increase the value of a DOF. The DOF value is, however, in proportion to λ/NA2, and therefore, when the wavelength is reduced and the numerical aperture NA is increased for improving the contrast, the DOF value is unavoidably reduced.
As described so far, a technique to simultaneously realize improvement of contrast by a method other than by reducing the wavelength and increasing the numerical aperture and improvement of a DOF without changing the wavelength λ and the numerical aperture NA has become significant.
In the most typical method for largely improving the contrast and the DOF, oblique incident exposure (off-axis illumination) is performed on periodical patterns provided on a photomask. However, the oblique incident exposure can exhibit a satisfactory effect merely when the patterns are arranged at a small period of λ/NA or less, and hence is not an effective method for refinement of arbitrary patterns. In order to make up for this weak point of the oblique incident exposure, there is a method using an auxiliary pattern (hereinafter referred to as the auxiliary pattern method).
Now, an auxiliary pattern method disclosed in Japanese Laid-Open Patent Publication No. 5-165194 (hereinafter referred to as the first conventional example) will be described. FIG. 35 is a plan view of a photomask used in the first conventional example. The photomask of FIG. 35 is used in a stepper for performing ⅕ reduction projection exposure. As shown in FIG. 35, a shielding film 11 of chromium is provided on a transparent glass substrate 10 working as a mask substrate. A first opening 12 corresponding to a main pattern (circuit pattern) to be transferred through the exposure is formed in the shielding film 11. Also, a pair of second openings 13 that correspond to an auxiliary pattern for improving the transfer accuracy of the main pattern and are not transferred through the exposure are provided in the shielding film 11 on both sides of the first opening 12. In this case, the first opening 12 has a width of, for example, 1.5 μm and each second opening 13 has a width of, for example, 0.75 μm. Also, the distance between the center of the first opening 12 and the center of each second opening 13 is, for example, 4.5 μm. In other words, in the photomask used in the first conventional example, an auxiliary pattern smaller than a circuit pattern, namely, a main pattern, is provided to be adjacent to the circuit pattern. In the auxiliary pattern method of the first conventional example, although the DOF is slightly improved, the effect equivalent to that attained by using the original periodic patterns cannot be attained.
Next, an auxiliary pattern method disclosed in Japanese Laid-Open Patent Publication No. 9-73166 (hereinafter referred to as a second conventional example), which is obtained by improving the first conventional example, will be described. FIG. 36 is a plan view of a photomask used in the second conventional example. As shown in FIG. 36, a main pattern 21 is provided on a transparent glass substrate 20 working as a mask substrate, and auxiliary patterns 22 are periodically arranged on the glass substrate 20 on both sides of the main pattern 21. The main pattern 21 is made from a multilayer film consisting of a lower low transmittance film and an upper shielding film (chromium film). Each auxiliary pattern 22 is made from the low transmittance film obtained by removing the upper shielding film from the multilayer film. At this point, the auxiliary pattern 22 made from the low transmittance film is not used for forming an unexposed portion of a resist (i.e., a resist pattern) through exposure. Accordingly, when the oblique incident exposure is performed with the auxiliary patterns 22 having low transmittance periodically arranged against the main pattern 21, the DOF can be improved.
Although the contrast and the DOF can be largely improved by using a phase shifter, but this effect can be attained merely in the case where a transparent portion (opening) and a phase shifter for transmitting exposing light at a phase difference of 180° with respect to the transparent portion can be arranged on both sides of a fine line pattern on a photomask. Accordingly, even when a phase shifter is used, the contrast and the DOF cannot be improved all over fine portions of an interconnect pattern of a general LSI.
Also, the contrast and the DOF can be largely improved in a completely periodic pattern by using the oblique incident exposure. However, this effect cannot be attained all over fine portions of an interconnect pattern including an isolated pattern of a general LSI. In this case, the DOF and the like can be slightly improved by using an auxiliary pattern (as in the first conventional example), but the effect is slight as compared with that attained in a completely periodic patterns. Also, when a pattern having low transmittance is used as an auxiliary pattern so as to improve the degree of freedom in the arrangement of the auxiliary pattern, the periodicity in the arrangement of patterns can be improved (as in the second conventional example). Also in this case, however, there arises the following problem: a substantial effect attained by the second conventional example is merely to ease processing of the auxiliary patterns because the auxiliary patterns can be made thick. In other words, with respect to the improvement of the contrast and the DOF, the second conventional example can attain the effect merely equivalent to that attained by the first conventional example (using a thin auxiliary pattern). This is because the improvement of the contrast and the DOF does not depend upon whether a mask pattern consisting of a main pattern and an auxiliary pattern is periodic but depends upon whether an image formed by using a mask pattern through exposure (i.e., an energy intensity distribution) is highly periodic.